The concept of implanting an intraocular lens as a replacement for an opaque crystalline lens of a human eye was suggested as early as 1766 by Cassanova in his memoirs. It has been only within the last thirty-five years or so, however, that theory and desire, have become a practical reality. In this connection, the first lens implantation is believed to have been carried out in 1949 by Dr. Harold Ridley at the Thomas Hospital in London. A lens was inserted into the posterior chamber of a woman about sixty years of age following cataract extraction. Dr. Ridley's early efforts achieved a degree of success and ophthalmic surgeons began implanting lenses composed of polymethylmethacrylate (PMMA) within the posterior chamber of the human eye following extracapsular removal of a cataract to restore binocular vision to patients.
Notwithstanding initial successes, drawbacks were also encountered. Early posterior chamber lens implantation reported incidences of dislocation, iris atrophy from pressure of the rim of a badly centered lens, and secondary glaucoma.
In addition to posterior chamber implantation, the attention of early surgeons also focused on the anterior chamber as a location for placement of an intraocular lens. Relatively rigid lenses were designed by Dr. Strampelli to be mounted within the anterior chamber and supported by the anterior chamber angle. A major disadvantage of rigid anterior chamber lenses was the danger it posed to the corneal endothelium and other delicate structures of the eye which are very sensitive to pressure or contact. The endothelium is a corneal cellular layer which may die upon touching and usually does not regenerate. More particularly, a lens that was too short would tend to move around inside the eye, twisting and turning on its axis, creating problems that were often complicated by endothelial corneal dystrophy. In contrast, a lens that was too long tended to distort the eye globe and damage angle structures.
Many of the difficulties encountered with rigid structures were alleviated, to a degree, by the deveIopment of flexible supports or loops by Dr. Dannheim. Even flexible loops, however, are subject to size limitations. Moreover, excessive flexibility or breakage of the supporting loops presented a danger to the iris and corneal endothelium. Additionally, flexible loop haptics were often designed with only three or four points of contact. Accordingly, eye tenderness was frequently encountered. In order to obviate eye tenderness some designs extended the length of contact of the flexible loop with the anterior chamber until a bearing ring was functionally created. When the loops were extended substantially around the periphery of the anterior chamber angle, however, interference was often encountered with the proper functioning of the sinus venosus sclerae (Schlemm's canal).
Although significant achievements were realized in the 1950s and 1960s, as late as 1968 there were still only about three choices for ophthalmologists desiring to implant intraocular lenses: a Choyce lens, an iris-plane lens and a four-loop Binkhorst lens. Within the last decade, however, lens design has expanded considerably and in 1982 approximately 175 intraocular lens designs were available from sixteen companies.
Currently in most intraocular lens designs, a PMMA lens is supported and held in place by a plurality of support strands or haptics. Each of the haptics is attached or secured to the peripheral lens body edge and each is flexible, that is to say each strand must be yielding under pressure, but must also have a memory retaining feature whereby the strand will return to its normal extended position or will automatically tend to do so once contact pressure has been released. Thus, the haptics have a spring-like quality and are normally composed of a biologically inert plastic material.
The haptics are attached to the lens body by any convenient or desirable means. One means is to have a hole provided in the lens periphery by boring, drilling or molding and to insert one end of the haptic into the hole and secure the haptic by an adhesive, interference or other mechanical means.
An arched distal end of the strand or loop provides a rounded haptic surface to contact the eye tissue once the lens is implanted. The longer and more gradual the arch, the less likely there will be any injury or trauma to the ciliary body tissue against which the arched strand abuts.
In the 1950s and 1960s nylon in various polymeric forms gained widespread popularity both as a loop material and as a fixation suture due to its strength, handling characteristics and believed durability. Despite initial success, cases of breakage and degradation of nylon were reported. Surface roughness and cracking in nylon sutures have been shown by scanning electron microscope. The hydrophilic tendencies of nylon led at least some investigators to believe that hydraulic degradation was at fault. In any event, nylon was discontinued as a haptic material by most manufacturers in 1979.
In the 1978-79 time frame, the advantages of polypropylene were recognized including low specific gravity and thus buoyancy support for a lens, nonabsorbability, relative inertness and stability, elasticity, minimal tissue reactivity, strength in the post-operative period, resistance to bacterial contamination and good tolerance by the patient. A proprietary FDA approved polypropylene is Ethicon brand "Prolene" supplied by Ethicon Pharmaceutical Company. Such material has superior qualities over other strand or loop materials such as gut, silk and nylon. The former materials, although flexible, do not achieve the desired buoyancy or spring-like memory retention qualities. The spring-like qualities of the strands which are continually urged to their greatest extension and are slightly compressed on their fully extended position maintain the lens body correctly centered in the ciliary space and posterior chamber. Thus, the springy haptics are biased and urged against the ciliary body to achieve desired centering and fixation of the intraocular lens.
Notwithstanding the advantages detailed above, in at least some quarters polypropylene haptics began to be investigated by clinical evaluation and laboratory research relating to the biocompatibility of the polypropylene when used in the intraocular lens loops or footplates. Scanning electron microscope findings of the morphology of haptics from removed intraocular lenses indicate possible alterations demonstrably connected to in-vivo changes. These alterations may be interpreted as resulting from a broad spectrum of situations, ranging from harmless manufacturing defects, to stress cracking, to possible progression toward actual loop degradation. In particular, an article titled "Biocompatibility of Implant Study" by Dr. David J. Apple et al. printed in American Intra-Ocular Implant Society Journal, Volume 10, Number 1, Winter 1984, addressed many of those problems. Widespread potential complications have been considered in view of observed in-vivo changes in the haptic material and the present invention comprehends prior concern over possible immune and inflammatory reactions, erosions into tissue, hemorrhage, or possible lens dislocation.
With the refinement of the art of lens design and manufacturing and with increased experience in ophthalmic surgery, surgeons' lens implantation is now one of the safest procedures in modern surgery. It has been reported that 496,000 or nearly one half million intraocular lenses were implanted in the United States alone between Feb. 1982 and Feb. 1983. It is envisioned by many that this figure will soon top one million per year. With the increased success in recent surgical developments, intraocular lenses now are being implanted in young adults, in children and in patients with borderline or low corneal endothelial cell counts, in monocular patients and in patients with various preexisting ocular and systemic diseases.
Preliminary findings indicate that changes in polypropylene haptic material may be time related and more severe changes are observed in highly metabolic tissue such as the ciliary sulcus. It is believed that induced irregularities in the loop surface cause exaggerated irritation due to mechanical rubbing against adjacent tissues leading to complications such as breakdown of the blood-aqueous barrier, release of prostaglandins or oxidating agents, or inflammation.
Haptic materials discussed above have included nylon and polypropylene both of which have been used successfully over the years in intraocular lenses. Other support materials include metal loops of various types but such structures have been found unacceptable because of several complications related to weight and fixation.
Some intraocular lenses employ haptic footplates in place of a loop or strand and these plates extend from the lens body, which is usually made of PMMA, and terminate in rounded or blunt shapes for seating in an eye chamber. Here the materials for the footplates have also included PMMA and most recently soft materials such as soft hydrogels or hydrophilic type such as 2-hydroxyethyl methacrylate, generally referred to as PHEMA.
As with the metal loops, the haptic footplates add extra material weight to the lens structure when compared to the loops or strand haptics.
Another intraocular lens available today is a PMMA lens body with haptics comprised of PMMA loop material. While these PMMA loops provide an excellent ocular prosthesis, the PMMA material is stiffer than polypropylene and can be quite difficult to remove when necessary. Additionally, some patients believe they can feel the pressure of PMMA loop material in an eye. This is especially prominent when a patient has a polypropylene loop intraocular lens in one eye, showing no discomfort, and reports a kind of "pressure feeling" in the other eye where an all PMMA loop intraocular lens resides.
Not only are different materials known for use in an intraocular lens but it has been known to use a different material for the haptics than for the lens itself. Specifically, U.S. Pat. No. 3,996,627 teaches an intraocularly implantable lens of glass wherein the haptics are constructed entirely of strands of plastic material, e.g., extruded Teflon, or of metal, e.g., platinum, titanium or tantalum wire. Thus, the lens and the haptics are composed of two different materials.
It is also known to use a coating of silicone rubber on a metal or plastic member. German Patent No. 25 56 665 teaches an intraocular lens where the haptic edge has an elliptic contour with a bulge at a secondary epice and wherein the optical portion is made entirely of silicone rubber. Methylsiloxanes or methylphenysiloxanes are the preferred materials for making the silicone rubber material, but it is also suggested to use siloxanes in which the remaining valences of silicone are saturated with propyl and phenyl groups. It is suggested that the optical portion and the haptic edge, surrounding the optical part, can be manufactured from one single piece of silicone rubber, while the haptics are made of metal or plastic and can be coated with a layer of silicone rubber.
U.S. Pat. No. 4,172,297 also discloses a lens of transparent material such as silicone rubber having a central lens body having front and rear ends, and two discs adapted to overlap the edge of the iris which surrounds the pupil with an annular space being formed between the discs and the space widening towards its outer edge for receiving the pupil edge of the iris.
In view of the relative satisfaction found with haptic materials currently in use today, amazingly little attention has been extended to the concept of coating the haptics in an attempt to achieve haptics of extended wearability.
It has recently been discovered that it is possible to alter only the surface of certain materials such as metals, ceramics, silicon and plastics using a process known as ion beam implantation (IBI). The process involves firing electrically charged atoms (ions) at a material to achieve a variety of results. These results include prolonged material wear, superior conductive (or insulating) properties, oxidation- and corrosion-resistance, reduction of surface friction and alterations in magnetism and other intrinsic properties.
In addition, because of their physical and/or chemical structure, many substances cannot be combined even though the properties of each may be extremely desirable when paired. An example would be the conductivity of metal with the corrosion-resistance of ceramic. Ion implantation allows such mixing to occur at the surface level of the material. Through a technique known as "sputtering", invisible beams of one or more materials are "sprayed" onto another material. The ions impregnate the surface, creating a new material with the attractive characteristics of all its component substances.
In the last few years, the use of ion beams to charge the surface properties of materials has gained increasing attention. Atoms of many ionizable elements have been excited to high speeds and shot into the surface of a variety of solid materials. Ionization is the process by which atoms or groups of atoms assume a net electrical charge by losing or gaining electrons. This process is known as on-beam processing or ion-beam implantation and its major use at the present time is in the semi-conductor industry, where silicon is doped to alter its electrical properties. The most recent developments are in the treatment of metal, ceramic, glass or polymeric surfaces to upgrade hardness, optical properties, corrosion resistance, electrical conductivity, and other characteristics. Such elements as nitrogen, argon, boron, platinum, rhodium, phosphorus, chromium, and polybdenum have been implanted in steel (and many of its alloys), titanium, and copper, as well as in glasses and ceramics.
Nitrogen implantation is the process most heavily researched and easiest to perform and it was one of the first commercial applications for ion-beam implantation. Generally, the nitrogen ions emerge from an ionization chamber as part of a 50-50 mixture of ions and charged molecules. The nitrogen ions make the material or substrate more durable, reduce surface flaws and minimize other defects, and prevent spalling (peeling away of the surface layer) and other types degradation. One example is nitrogen treatment of surgical bone implants. In many such prostheses, this treatment reduces wear rates by a faction of 400 or more. Implants have also been treated with carbon with good results. The surface wear has been reduced to negligible levels and greatly extends the useful life of the implant.
The greatest research effort has been in the area of metals as previously indicated. At the present time little emphasis has been placed on the treatment of polymers with ion beam implantation. For instance, it has been determined that the electrical conductivity of polyacetylene may be enhanced by bombarding it with ionized conducting metals. In addition, injection molds, nozzles and bushes used in the plastics industry have been treated with nitrogen. Abrasive fillers in plastic material present a serious erosive problem in plastic injection molding operations. Thus, nitrogen implantation has been used for protecting the expensive mold surfaces. However, the biomedical applications of polymers coated by ion implantation have not received any attention.
The subject invention goes beyond anything offered by the prior art especially the prior art covering intraocular lenses and is directed toward an improved haptic loop and makes available to the surgeon and wearer an intraocular lens having superior material characteristics thereby extending the usable life of the haptic surface and ensuring the patient of many years of satisfactory service from an intraocular lens.
The problems suggested in the preceding are not intended to be exhaustive, but rather are among many which may reduce the effectiveness and user satisfaction of prior haptic carrier devices. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that haptic loop material appearing in the past will admit to worthwhile improvement.
The subject invention is directed toward an improved haptic providing for enhanced haptic longevity and biocompatibility without detracting from the advantageous features of prior devices. More particularly, the present invention is directed to the use of a coating process for haptics which would protect the haptics and make them resistant to corrosion or oxidation thereby extending their useful life.